Controlled C–H bond activation leads to orthometalation and ring-hydroxylation in Ni(II) and Pd(II) complexes of a common tridentate azophenyl-salicylaldimine ligand

Abstract

Using a common tridentate azophenyl-appended salicylaldimine ligand in its deprotonated form (L4)–, two classical coordination complexes [Ni(L4)2] (1) (NiIIO2N2N'2 coordination) and [Pd(L4)Cl]•CH2Cl2 (2) (PdIIONN'Cl coordination), two cyclometalated complexes [M(L4*)] (M = Ni 3 and Pd 4; MIIONN'C coordination), and two azophenyl ring-hydroxylated complexes [M(L4–O)] (M = Ni 5 and Pd 6; MIIONN'O' coordination), obtained due to selective oxidation of MII–C bond, have been synthesized. Complex 1 is paramagnetic (S = 1) but all other complexes are uniformly diamagnetic (S = 0). For 1–6, absorption spectral and redox properties have been investigated, along with single-crystal structural analysis. The regiospecific aryl ring-hydroxylation of M−C bonds in 3 and 4 are achieved due to H2O2 and m-CPBA oxidation, respectively, affording 5 and 6. Coulometically-generated one-electron oxidation and one-electron reduction have been done on 1–6, and the nature of the resulting species has been probed by EPR and absorption spectral studies. While oxidation of 1 and 3 generate Ni(III) species, it is a resonance hybrid between Ni(III) ⇔ Ni(II)-O(phenoxyl radical) species for 5. On the other hand, 2, 4, and 6 generate ligand radical species. Reduction of 1–6 generates ligand radical species uniformly. Density functional theory (DFT) calculations at the B3LYP level of theory have been done to extract information about the electronic structure of the complexes. Time-dependent (TD)-DFT calculations have been done to shed light on the origin of observed absorption spectra. Plausible mechanisms have been proposed for the observed orthometalation and ring-hydroxylation reactions.